Mg Doped Perovskite LaNiO3 Nanofibers as an Efficient Bi-functional

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Mg Doped Perovskite LaNiO3 Nanofibers as an Efficient Bi-functional Catalyst for Rechargeable Zinc-air Batteries Juanjuan Bian, Rui Su, Yuan Yao, Jian Wang, Jigang Zhou, Fan Li, Zhonglin Wang, and Chunwen Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02183 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Applied Energy Materials

Mg Doped Perovskite LaNiO3 Nanofibers as an Efficient Bi-functional Catalyst for Rechargeable Zinc-air Batteries Juanjuan Bian,1,2 Rui Su,3* Yuan Yao,4 Jian Wang,5 Jigang Zhou,5 Fan Li,6 Zhong Lin Wang1,2,7,8* and Chunwen Sun1,2,7*

1CAS

Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China 2School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China 3Innovative Center for Advanced Materials, Hangzhou Dianzi University, Hangzhou 310018, China 4Laboratory for Advanced Materials and Electron Microscopy, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 5Canadian Light Source Inc., University of Saskatchewan, Saskatoon, SK S7N 2V3, Canada 6Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China. 7Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, China 8School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA *

Corresponding authors.

Tel.:

+86-10-82854648,

fax:

+86-10-82854648.

Email:

[email protected]

[email protected] (Z. L. Wang), [email protected] (C. Sun)

1

ACS Paragon Plus Environment

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Su),

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Rational design of efficient and durable bifunctional catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is important for rechargeable zinc-air batteries. Herein, Mg doped perovskite LaNiO3 (LNO) nanofibers (LNMO NFs) were prepared by a facile electrospinning method combined with subsequent calcination. LNMO NFs show a more positive half-wave potential of 0.69V and a lower overpotential of 0.45 V at a current density of 10 mA cm-2 than those of the pristine LNO NFs. As an air electrode for zinc-air battery, the cell with LaNi0.85Mg0.15O3 NFs catalyst is able to deliver a high specific capacity of 809.9 mAh g-1 at a current density of 5 mA cm-2. It also shows an excellent cycling stability over 110 h at a current density of 10 mA cm−2. DFT calculation results demonstrate that the LNMO surface binds oxygen stronger than LNO, which contributes to enhanced OER activity as observed in our experiments. The results indicate that LNMO NFs is an efficient and durable bifunctional catalyst for zinc-air batteries.

Keywords: Mg doped LaNiO3 nanofibers, electrospinning, bi-functional catalyst, density functional theory calculation, zinc-air batteries.

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INTRODUCTION With increasing consumption of fossil energy and accelerating industrialization, the demand for renewable energy is ever increasing.1,2 Since renewable energy like wind and solar energy is random and difficult to connect with the electrical grid, it is very urgent to develop new energy storage technologies.2-4 Metal-air batteries have attracted much attention due to their high energy density.4,5 In particular, zinc-air batteries are noticeable with merits of low cost, high capacity, stabled discharge voltage, good safety, environment friendly, etc..6-12 Air electrode catalyst have great effects on the performances of the batteries such as output power and life.4,8 Therefore, efficient catalysts with high efficiency and low cost have been pursued by researchers.13,14 Perovskite oxides with advantages of low cost, stable performance and low electric resistance at room temperature show high catalytic activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER),14-17 which are one of the promising bifunctional oxygen electrocatalysts.15,18 Recently, it is reported that semimetallic oxide LaNiO3- has low charge-transfer energy with a Tafel slope of 60 mV dec-1.

19

Moreover, one dimensional nanofibers with a high

aspect ratio and porosity can maximize the catalytic reaction sites and facilitate the diffusion of electrons and reactants.16 Herein, we prepared perovskite LaNiO3 and LaNi0.85Mg0.15O3 nanofibers (denoted as LNO and LNMO NFs, respectively) via an electrospinning method combined with subsequent heating treatment used as oxygen electrocatalysts in zinc-air batteries for the first time. By doping 15% Mg2+ into the B site of LaNiO3, the obtained LaNi0.85Mg0.15O3 NFs show the best electrochemical performance.20 LaNi0.85Mg0.15O3 NFs exhibits higher catalytic activity toward ORR and OER than that of LaNiO3 NFs in 0.1M KOH solution. LNMO NFs has a more positive half-wave potential (0.69 V) than LNO NFs (0.63 V), while it has a lower overpotential of 0.45 V at a current density of 10 mA cm-2 than that of LNO NFs (0.55 V). Moreover, LNMO NFs show smaller Tafel slopes of 105 mV dec-1 for ORR and 95 mV dec-1 for OER, respectively. 3



As an air electrode for zinc-air batteries, the cell with LNMO NFs catalyst is able to deliver a high specific capacity of 809.9 mAh g-1 at a current density of 5 mA cm-2. It also shows an excellent cycling stability over 110 h at a current density of 10 mA cm−2.

EXPERIMENTAL SECTION Synthesis of catalysts. The LNO nanofibers were synthesized by an electrospinning method combined with subsequent heating treatment. In a typical process, firstly, 1.083 g La(NO3)3·6H2O and 0.622 g Ni(Ac)2·4H2O were added into 10 mL N,N-dimethylformanide (DMF) under a vigorous magnetic stirring for 15 min until a homogeneous solution is formed. Then, 1.049 g of polyvinylpyrrolodone (PVP) was added slowly into the above mixed solution for 2 h under stirring. Then, the obtained solution was loaded into a plastic syringe equipped with a 19-gauge metal nozzle made of stainless steel. The feed rate of the solution was kept at a flow rate of 0.2 mL h-1. The distance between the collector and the needle tip was 20 cm. The applied voltage was fixed to 20.0 kV, and the electrospinning fibers were collected on a piece of aluminum foil. The as-spun nanofibers were collected and dried at 80oC for 12h and then calcined at a heating rate of 2oC min-1 in air and held at 700oC for 2 h. The obtained product is denoted as LNO NFs. The synthesis process of LNMO is similar to that of the LNO NFs. In detail, 1.083 g La(NO3)3·6H2O、0.529 g Ni(Ac)2·4H2O and 0.096 g Mg(NO3)3·6H2O were added into 10 mL DMF under vigorous magnetic stirring for 15min until a homogeneous solution was formed, then 1.049 g PVP was added slowly into the above mixed solution for 2 h under stirring. Then, the solution was loaded into a plastic syringe equipped with a 22-gauge metal nozzle made of stainless steel. The as-spun nanofibers were collected and dried at 80oC for 12h and were calcined at a heating rate of 2oC min-1 in air at 700oC and held for 2 h at the target temperature. The obtained product is denoted as LNMO NFs. Materials characterization. Powder X-ray diffraction (XRD) analysis was 4


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characterized on a PANalytical X’Pert powder diffractometer with Cu Kα radiation (  = 1.5418, 40kV, 40 mA) in the 2  range of 10−80°. The morphology was characterized by transmission electron microscopy (TEM, Tecnai G2 F20) and field-emission scanning electron microscopy (FESEM, SU8020). Nitrogen sorption isotherms were measured at 77 K with a Quantachrome ASIQC0R100-3 analyzer. Before measurements, the samples were degassed under vacuum at 150 oC for 6h. The pore volume and pore diameter distribution were derived from adsorption branches of the isotherms by the Barrett-Joyner-Halenda (BJH) model. X-ray absorption near-edge spectra (XANES) were obtained on the scanning transmission X-ray microscope (STXM) at the SM beamline of the Canadian Light Source (CLS). Powder samples were dispersed in methanol with brief sonication and then deposited onto Si3N4 window substrate for transmission STXM measurement. STXM spectro-microscopic data (i.e. image stacks) at the O K-edge and Ni L-edge were acquired at the same selected sample region. STXM XANES spectra were extracted from the sample areas with uniform and suitable thickness by software aXis2000 (http://unicorn.mcmaster.ca/aXis2000.html). Electrochemical measurements. Electrocatalyst inks was prepared by dispersing 4.5 mg of catalyst /Super P (1:1, mass ratio) in 2 mL alcohol and 1mL 0.5 wt% Nafion by sonication treatment. Then 20 L ink was dropped on a polished glassy-carbon (GC) rotating disk electrode. The loading of the catalysts is 0.153 mg cm-2. The electrochemical measurements were performed with a three-electrode configuration in 0.1M KOH electrolyte to test ORR and OER performance of the catalysts. The commercial Pt/C catalyst/Super P (1:1, mass ratio) and RuO2 catalyst/Super P (1:1, mass ratio) were also tested using the same procedure for comparison. Assembly of rechargeable zinc-air battery. The zinc-air batteries were assembled with a home-made cell. A polished zinc foil was used as the anode. The air cathode consist of catalyst, Super P, and 5% Nafion solution with a mass ratio of 7:7:6 to form a coating on the nickel foam with a loading of 1 mg/cm2. The other side of nickel foam was coated with an air diffusion layer consisting of activated carbon and PTFE. 5



The cathode and anode were separated by a polypropylene/polyethylene (PP/PE) membrane and the electrolyte is composed of 6M KOH and 0.2M zinc acetate.

DFT calculations. All calculations are performed by using the Vienna ab initio simulation package (VASP).21 The electron wave function is expanded by planewave basis with a kinetic energy cutoff of 400 eV. The projector augmented wave method is used to describe the core-valence interaction. The K-points mesh is set to 2×2×1. The periodic images are separated by vacuum layers larger than 15Å to remove interactions of periodic images. We use the BEEF-vdW exchange correlation function22 which contains non-local van der Waals correction and give accurate chemisorption and physisorption energies for various systems. The strong correlation effect of Ni 3d orbitals is considered by the DFT+U method with a U value of 6.2 eV.

RESULTS AND DISCUSSION LNMO and LNO NFs were prepared by electrospinning and subsequent calcination as schematically illustrated in Figure 1a. The phase and crystal structure of the products were examined by powder X-ray diffraction (XRD). Figure 1b shows XRD patterns of the LNO and LNMO samples. All the diffraction peaks of the as-prepared LNO product (black line) can be indexed to a well-crystallized perovskite-type oxide, which is well consistent with LaNiO3 (JCPDS No. 34-1077). The diffraction peaks of LNMO (red line) are slightly different from those of LNO. The main peak at 32.82o is slightly shifted to the left since the ionic radius of Mg2+ (0.57 Å) is larger than that of Ni2+ (0.55 Å).23 The porous structure and pore size have been investigated by nitrogen adsorption -desorption measurements. Figure 1c displays the nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of the LNMO and LNO NFs samples. The specific surface areas of LNMO NFs and LNO are 21.192 m2 g-1 and 11.935m2 g-1. From the inset image, it can be seen that the average pore diameters of 6


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the LNMO and LNO are 175 and 50 nm, respectively. The size and morphology of the products were examined by field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. Figures 1d and 1e show SEM images of the as-electrospun product of LNO while Figures 1f and 1g display SEM images of the calcined LNO and LNMO NFs at 700oC. It can be seen that the nanofiber morphology is well kept after calcination. Mg doping did not change the morphology. After calcination, there is some shrinkage of the nanofibers. This is due to lots of PVP existing in the nanofibers precursor, which evaporates during calcination. One-dimensional nanofiber structure can improve electronic conductivity and facilitate mass transfer, and thus enhance the reaction kinetics of the catalyst.16.24-26 The porous nanofibers obtained by electrospinning play an important role in improving the ORR and OER properties of the material since the one-dimensional structure is favorable for the transport of electron.16

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Figure 1. (a) Schematic illustration of the synthesis processes of LNMO. (b) XRD patterns of the LNO and LNMO NFs samples. (c) Nitrogen adsorption − desorption isotherms and pore size distribution (inset of c) of the two samples. (d) SEM images of the as-electrospun product of LNO. (e) SEM images of the as-electrospun product of LNMO. (f) SEM images of the LNO NFs after calcination at 700oC. (g) SEM images of the LNMO NFs after calcinations at 700oC . 8


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TEM was used to characterize the microstructure of the obtained products. Figure 2a shows the overview TEM image of LNMO NFs. It indicates that the LNMO NFs consist of nanoparticles, forming a porous structure. The marked interplanar spacing in Figure 2b is about 0.213 nm, which corresponds to the (202) lattice planes of LNMO NFs. Figure 2c shows an annular dark-field (ADF) scanning transmission electron microscopy (STEM) image of the LNMO NF. To further clarify the chemical composition of the nanofiber, Figure 2d-g presents the corresponding energy dispersive X-ray spectrometry (EDX) elemental mappings of La 、 Ni 、 Mg and O, respectively. All the elements are uniformly distributed in the LNMO nanofibers. Figure S1 and S2 show the overview TEM image and STEM of LNO NFs.

Figure 2. (a) TEM image of LNMO NFs. (b) HRTEM images of a LNMO NF. (c) ADF STEM image of LNMO NF. (d-g) The corresponding elemental mappings of La, Ni, Mg and O of the LNMO NF. 9



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Doping Mg in LNMO NFs is also characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figure S3a, the peak around 50eV for LNMO is assigned to Mg 2p while no peak is observed for LNO, indicating that Mg is indeed doped into LNMO NFs. In Figure S3b, the deconvolved peaks at

854.3 eV and

856 eV can be assigned to Ni2+ and Ni3+, respectively. 20 It can be seen that the ratio of Ni3+to Ni2+ increases in LNMO NFs due to Mg doping. The ORR and OER catalytic activities of the LNO and LNMO NFs were studied by rotating disk electrode (RDE) in O2-saturated 0.1 M KOH solution and compared with that of the commercial Pt/C and RuO2 catalyst. Figure 3a shows the cyclic voltammetry (CV) curves of the LNO NFs, LNMO NFs and Pt/C catalysts tested in O2- and N2-saturated 0.1M KOH solution. It can be seen that LNMO NFs show more positive ORR onset potential than that of the LNO NFs, although it is negative compared with the commercial Pt/C catalyst. Linear-sweep voltammograms (LSV) curves is displayed in Figure 3b. LSV was operated at a rotation rate of 1600 rpm and sweep rate of 5 mV s-1 in O2-saturated 0.1M KOH. From the LSV curve, it can be seen that for the ORR performance, the half-wave potential of the LNMO NFs, LNO NFs and commercial Pt/C catalyst are 0.69V, 0.63V and 0.84V, respectively. For the OER performance, LNMO exhibits a low overpotential of 0.45V at a current density of 10 mA cm-2 which is less than that of LNO NFs and close to the commercial RuO2 , indicating that LNMO NFs have a good OER performance. Figure 3c show ORR polarization curves of different catalysts. As shown in Figure 3d, the LNMO NFs catalyst shows a Tafel slope of 105 mV dec-1, which is smaller than that of the LNO NFs catalyst (165 mV dec-1), suggesting that the LNMO NFs catalyst has a favorable ORR kinetics.20, 27 Figure 3f shows the Tafel slopes of the OER curves, indicating that the LNMO NFs catalyst exhibits a much lower Tafel slope of 95 mV dec-1 than that of the LNO NFs and the commercial RuO2 catalysts. From Tafel slopes, it can be seen that LNMO NFs catalyst possesses more favorable kinetics in both ORR and OER compared with those of the LNO NFs. Moreover, the oxygen electrode performance and reversibility are usually evaluated by the deviation of OER from 10


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ORR metrics, i.e., potential at a current density of 10 mA cm-2 in OER and the half-wave potential in ORR.27 It can be seen that the LNMO NFs catalyst shows a high oxygen electrode activity with lower E=Ej=10-E1/2, about 0.99 V. The electrochemical active surface area (ECSA) was measured by double layer capacitance (CDL) to disclose the underlying reason of the superior activity of LNMO NFs catalyst. Figure S4 and S5 show that LNMO NFs catalyst has bigger electrochemically active surface area of 2.69 cm2 than that of LaNiO3 of 1.30 cm2. It is noted that all the electrochemical results are iR-corrected to compensate for the resistance of the electrolyte solution. The corrected EiR=E- iR, where i is the current and R is the ohmic resistance of electrolyte solution. 28

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Figure 3. (a) CV curves of LNO NFs, LNMO NFs and the commercial Pt/C catalyst on glassy carbon electrodes in O2-saturated (red solid line) and N2-saturated (black dotted line) 0.1M KOH (sweep rate: 50 mV s-1). (b) LSV curves of LNO, LNMO, Pt/C and RuO2 in O2-saturated 0.1M KOH (Rotation rate: 1600 rpm; sweep rate: 5 mV s-1). (c) ORR polarization curves. (d) Tafel slope of ORR curves. (e) OER polarization curves.

(f) Tafel slopes of OER curves. Catalyst loading is 0.15 mg

cm-2 for all catalysts.

In order to further quantify the ORR kinetic properties of LNO and LNMO, we 12


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performed rotating ring-disk electrode (RRDE) measurements as shown in Figure 4a. It is obviously that LNMO possesses superior Eonset and E1/2 to LNO. LNMO also has higher disk current for O2 reduction and lower ring current for H2O2 oxidation .These results are consistent with the RDE results (Figure S6). From Figure 4b we could obtain that average numbers of electron transform is 3.4 for LNMO which were higher than LNO (3.0), and LNMO also generates less peroxide. We also tested the stability of LNO and LNMO NFs through chronoamperometric method at constant potential for the ORR and OER performance, as shown in Figure S7 . It is clear that the LNMO NFs possesses the superior stability to LNO NFs.

Figure 4. (a) RRDE measurements of LNO and LNMO NFs in 0.1M KOH. (b) Percentage of peroxide yield and electron numbers of LNO and LNMO NFs.

Table 1 presents the ORR and OER performance of the LNMO NFs, which is also compared with the results of other catalysts reported in the literatures. The results indicate that LNMO has excellent ORR and OER performances and faster electrochemical kinetics in terms of lower onset potential, higher oxygen electrode activity with smaller E and smaller Tafel slope.

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Table 1. Performance comparison of ORR and OER with different catalysts Catalyst

Eonset

E1/2

Ej=10

ΔE

Tafel

(V)

(V)

(V)

(V)

dec-1)

slope(mV

ORR

OER

Ref.

LaMn0.7Co0.3O3

0.88

0.73

1.82

1.09

110

151

29

LaMnO3

0.78

0.62

>2

>1.38

110.2

154

30

SrNb0.1Co0.7Fe0.2O3–δ

˗

˗

1.73

˗

˗

76

31

Ba0.5Sr0.5Co0.8Fe0.2O3–δ

˗

˗

1.74

˗

˗

94

32

Cu-N-C/graphene

0.8

0.65

>2

>1.35

76

˗

33

NPMC

0.94

0.85

1.95

1.1

˗

˗

34

BaTiO3

0.82

0.68

>2

>1.32

˗

˗

35

Co3O4/2.7Co2MnO4

0.8

0.68

1.77

1.09

˗

˗

36

graphene/SWCNT

0.76

0.65

1.74

1.09

˗

97

37

0.82

0.69

1.68

0.99

105

95

This work

hybrids LaNi0.85Mg0.15O3

In order to characterize the feasibility of LNMO and LNO NFs as bifunctional catalysts for metal-air batteries, these catalysts were examined with home-made zinc-air batteries. Figure 5a shows galvanostatic discharge measurements of LNMO NFs at current densities of 5 mA cm-2 and 10 mA cm-2, respectively. The specific capacities of the battery with the LNMO NFs as air electrode are estimated to be 809.9 mAh g-1 and 786.1mAh g-1 at current densities of 5 mA cm-2 and 10 mA cm-2, respectively. The capacities were calculated based on the mass of the consumed zinc foil. The specific capacities of the battery with the LNO NFs as air electrode are shown in Figure S8. Figure 5b shows the power density and polarization plots of the two-electrode primary ZABs with LNMO NFs and LNMO NFs catalysts. From the polarization curves, it can be seen that the cell with LNMO NF catalyst shows a power density of 45 mW cm-2, which is higher than that of the cell with LNO NFs (22 mW cm-2). Moreover, the cell with LNMO NFs catalyst can run at a larger current 14


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density (150 mA cm-2) while the cell with LNO NFs shows stably only at 65 mA cm-2. In order to characterize the cycling performance of the battery, a galvanostatic charge and discharge test was performed at a current density of 10 mA cm-2 with each cycle of 30 min, as shown in Figure 5c. It can be seen that the cell with LNMO NFs can work stably. The final charge and discharge potentials of LNO reach 2.4V and 0.8 V after 75 hours, respectively, whereas the initial values are 2.3V and 1.0 V. For the cell with LNMO NFs catalyst, it can be seen that the final charge and discharge potential of the cell are 2.1V and 0.8 V after 110 hours, whereas the initial values for charge and discharge potential are 2.1 V and 1.18 V. The long-term cycling performance of Pt/C and RuO2 is shown in Figure S9. Figure 5d displays two home-made zinc-air cells with LNMO NFs as the air electrode connected in series to power a light-emitting diode (LED) screen.

Figure 5. (a) Specific discharging capacity plots at 5 mA cm-2 and 10 mA cm-2 of the cell with LNMO NFs. (b) Discharging polarization curves and power density of the ZABs with two different catalysts. (c) Long-term cycling performance of the cells with LNO NFs (black line) and LNMO NFs (red line) as the air cathode at a charging 15



and discharging current density of 10 mA cm−2. (d) Photograph of a LED screen powered by two zinc–air cells connected in series with LNMO NFs as the air cathode.

Table S1 presents the performance of the LNMO NFs as air electrode for zinc-air batteries, which is also compared with the results of other catalysts reported in the literatures. The cell with the LNMO NFs catalyst shows better performances, including a higher specific capacity (809.9 mAh g-1 ) and long cycling stability (>110 h). As an air electrode for zinc-air batteries, LNMO NFs shows excellent durability. The improved performance is ascribed to the following aspects: (1) Nanofibers with one-dimensional structure promote the transport of electrons; (2) Doping Mg2+ at B site enhances the catalytic activity of the LNO; (3) Larger surface area in LaNi0.85Mg0.15O3 nanofibers is good for oxygen adsorption and transport.

Scanning transmission X-ray microscope (STXM) based X-ray absorption near-edge spectra (XANES) were used to explore the structural difference of the two samples. Figure 6a shows the O K-edge of LaNiO3 (black line) and LaNi0.85Mg0.15O3 (red line). Compared the intensities of peaks at ~528 and ~544 eV, the latter decreases, indicating a slightly lower Ni-O covalence for Mg doped LNM NFs, which is beneficial for O2-/OH- displacement and OH- regeneration, because the strong B-O covalence in perovskite ABO3 is not good for catalytic activity.38 At the same time, the XANES spectra of Ni L-edge also show similar information. Generally, the presence of Ni2+ in LaNiO3 leads to the number of electrons eg > 1, which decreases its ORR catalytic activity.38 However, it can be seen that the intensity of the peak at 853 eV of the LNMO increases, which indicates a little bit increase of Ni oxidation (Figure 6b). This means an increase of the ratio of Ni3+/Ni2+ and could result in eg electron occupation approaches to 1 after Mg doping. The eg-filling electron of ~1 will result in excellent ORR and OER performance.39 In summary, XANES result is consistent with the improved ORR and OER performance of Mg doped LaNiO3 NFs. 16


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Figure 6. (a) O K-edge and (b) Ni L-edge of STXM-XANES spectra for LaNiO3 NFs and LaNi0.85Mg0.15O3 NFs.

First-principles calculations were further performed to gain insight into the enhanced catalytic properties of LNMO NFs. The Ni sites of LaNiO3 (001) surface is taken as reference reaction local environment. Mg alloying effect is considered by replacing one bulk Ni atom and a subsurface oxygen vacancy is introduced to model the oxygen vacancy formation due to the alloying Mg ions. Free energies of all adsorption sites are calculated with a surface coverage of 0.25 monolayer (Figure S10). By employing the computational hydrogen electrode model, the reaction free energy path is computed at zero potential and pH=0 condition following a four-electron ORR/OER path. The potential and pH effects are then included by shifting the free energies accordingly.40 Free energies of individual and adsorbed molecules are computed with both zero-point energy and entropy changes considered. The entropy values of individual molecules are obtained from the JANAF table.41 The zero-point energy and entropy of adsorbed groups are computed by vibration analysis. Due to the bad description of O=O double bonding in DFT calculations, the O2 molecule energy is corrected by matching the experimental water formation free energy. The reaction free energy paths on the LNMO surface at different electrode potentials are shown in Figure 7. At U=0 V and pH=0 the whole ORR reaction is downhill. At the equilibrating potential (U=1.23 V), the ORR reaction is now limited 17



by the formation of OOH* adsorbates while the OER reaction is limited by the reduction of surface OH* adsorbates. Comparing to LNO surface, the adsorption energy of surface oxygen is slightly reduced which contributes to a 0.07 V reduction of the OER overpotential. Since the ORR path is not limited by the third electron transfer step, the overpotential of ORR is almost not changed. On the other side, the pH of water solution also contributes to the reaction free energies and is included as thermodynamic corrections. The corrected paths are shown in Figure S11 for different pH values which show its good activity in wide pH range. We note that in our simulation the pH value would not alter the overpotentials as the rate limiting step is not altered by different pH value. In summary, our simulation shows that the LNMO surface binds oxygen stronger than LNO which contributes to enhanced OER activity as observed in our experiments.

Figure 7. Computed reaction free energy diagram for ORR/OER reactions on LNMO 001 surfaces. The equilibrating and minimum potential path of ORR and OER are 18


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also presented for LNMO (color solid) and LNO (black dotted), respectively.

CONCLUSIONS In summary, perovskite LaNi0.85Mg0.15O3 NFs was prepared by a facile electrospinning and subsequent calcinations. LaNi0.85Mg0.15O3 NFs show superior catalytic activity for ORR and OER to LaNiO3 in 0.1M KOH, it exhibits a more positive half-wave potential (0.63 V) and a lower overpotential of 0.45 V at a current density of 10 mA cm-2 than that of LaNiO3 NFs. As an air electrode for zinc-air battery, the cell with LaNi0.85Mg0.15O3 NFs catalyst is able to deliver a high specific capacity of 809.9 mAh g-1 at a current density of 5 mA cm-2. It also has an excellent cycling stability over 110 h at a current density of 10 mA cm−2. DFT calculation shows that the LNMO surface binds oxygen stronger than LNO, which contributes to enhanced OER activity as observed in the experiments. LaNi0.85Mg0.15O3 NFs are a promising bifunctional catalyst for rechargeable zinc-air batteries with high catalytic performance, stable battery cycle performance and a high utilization value.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM image, XPS spectra of LNO and LNMO, Electrochemical stability , Performance comparison of Zn–air batteries with Pt/C and RuO2, Specific discharging capacity plots of the cell with LNO NFs, Long-term cycling performance at a charging and discharging current density, Optimized structures of clean and adsorbed LNMO 001 surfaces, and Free energy diagram for LNMO 001 at different pH values.

ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Key R & D Project 19



from Ministry of Science and Technology, China (2016YFA0202702), the National Science Foundation of China (Nos. 51372271, 51672029, 51602025 and 51474009) and Beijing Municipal Science & Technology Commission (Z171100000317001).

Conflict of Interest The authors declare no conflict of interest.

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Table of Contents (TOC) Perovskite LaNi0.85Mg0.15O3 nanofibers were prepared by an electrospinning technique followed by heat treatment and used as an excellent catalyst for rechargeable zinc-air batteries. The cell with LaNi0.85Mg0.15O3 NFs catalyst is able to deliver a high specific capacity of 809.9 mAh g-1 at a current density of 5 mA cm-2. It also shows excellent cycling stability over 110 h at a current density of 10 mA cm−2.

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Mg Doped Perovskite LaNiO3 Nanofibers as an Efficient Bi-functional

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